U.S. Pat. No. 3,893,869, assigned to RCA, discloses a cleaning system wherein very high frequency energy is employed to agitate a cleaning solution to loosen particles on the surfaces of semiconductor wafers. Maximum cleanliness for such items is desired in order to improve the yield of acceptable semiconductor chips made from such wafers. This cleaning, system has become known as megasonic cleaning, in contrast to ultrasonic cleaning in view of the high frequency energy employed. Ultrasonic cleaners generate random 20-40 kHz sonic waves that create tiny cavities in a cleaning solution. When these cavities implode, tremendous pressures are produced which can damage fragile substrates, especially wafers. Megasonic cleaning systems typically operate at a frequency over 20 times higher than ultrasonics, and consequently, they safely and effectively remove particles from materials without the side effects associated with ultrasonic cleaning.
A number of improvements have been made to the system as initially outlined in the above-referenced patent, and several companies are now marketing such cleaning apparatus. One of these is Verteq, Inc. of Anaheim, Calif., the assignee of the invention disclosed and claimed in this document.
One of the major improvements that helped make the Product a commercial reality concerns the design of the transducer array which converts electrical energy into sound waves for agitating the cleaning liquid. The transducer array is perhaps the most critical component of the megasonic cleaning system. The transducer array which has been developed over a number of years and is currently being marketed by Verteq is mounted on the bottom of the process tank close to the components to be cleaned so as to provide powerful particle removal capability. The transducer array includes a strong, rigid frame suitable for its environment, with a very thin layer of tantalum, which is a ductile acid-resisting metallic element, spread over the upper surface of the frame.
A pair of spaced rectangular ceramic transducers are positioned within a space in the plastic frame and bonded by electrically conductive epoxy to the lower side of the tantalum layer extending over the space in the frame. The transducer has a coating of silver on its upper and lower faces that form electrodes. RF (radio frequency) energy approximately 800 kHz is applied to the transducer by connecting one lead to the lower face of the transducer and by connecting the other lead to the layer of tantalum which is electrically conductive and which is in electrical contact with the upper silver coating of the transducer.
While megasonic cleaning systems employing this transducer array have enjoyed commercial success, improvements are needed. Foremost, it is highly desirable that the life of the transducer array be extended so as to reduce the cost of repair and replacement, and more importantly, to avoid interruptions in the processing of components by such cleaning apparatus. The cost of the overall system, which includes equipment for handling the cleaning solutions and further includes computerized controls, may exceed $25,000. Accordingly, it is not practical for users to keep an entire spare system, and a repair or replacement capability is not always readily available when needed.
Perhaps the most frequent failure in the transducer array concerns the bonding between the layer of tantalum and the upper silver coating on the transducers. Over a period of time, the vibration of the components will result in small bubbles or spaces in the epoxy bonding layer between the transducer and the tantalum sheet. Heat produced by the high energy is not as readily conducted away from these minute spaces as it is in the surrounding interconnection, with the result that hot spots eventually occur causing the bonding agent to further break down. Such heat eventually damages the thin tantalum layer. Moreover, as the hot spots increase in number and size, the effectiveness of the focused energy provided by the transducer array gradually declines such that the cleaning operation is less effective. Because of the hot spot problem, great care is taken in bonding the thin tantalum sheet to its support structure; however, this is a difficult task resulting in low productivity. After the bonding operation, small bubbles or imperfections can actually be felt by hand through the tantalum layer. If these are detected, the product is scrapped.
A number of efforts have been previously made to improve this situation. One company has greatly increased the thickness of the tantalum layer, apparently on the expectation that the greater thickness would better dissipate the heat build-up of hot spots, if they should start to occur. Further, a thicker layer adds structural strength to the assembly, which would help overcome an additional problem of the existing arrays concerning their durability. However, in addition to increasing the cost the thicker layer of tantalum does not appear to transmit the megasonic energy as effectively as the thin layer.
Another attempted approach was to use vitreous carbon instead of the thin layer of tantalum, in that such material is also electrically conductive and can withstand acid and other cleaning solutions, being particularly durable and hard. However, this approach was not successful due to the difficulty of fabricating vitreous carbon in a thin, smooth plate-like layer, as is done with tantalum.
Stainless steel has been used as an energy transmitting element with transducers being bonded to it, but it is not nearly as good as tantalum with regard to chemical inertness and contaminates, and with regard to mechanical erosion or stability.
It was also believed that the material should be electrically-conductive so as to facilitate electrical connection to the transducer conductive layer to which it is bonded. This requirement, of course, eliminated many materials from consideration.
The need for an improved solution to this problem of increasing the life of the transducer array has thus continued, and it is an object of the present invention to provide such an improvement.